Piezo-resistive bridge pressure sensors are used in a wide variety of applications including automotive, industrial, medical, and environmental applications. Such sensors typically include a silicon diaphragm incorporating an implanted piezo-resistive Wheatstone bridge. The applied pressure bends the diaphragm and imbalances the bridge, producing a differential, ratiometric output signal that is proportional to the product of the change in resistance caused by the pressure and the bridge excitation voltage.
In high performance pressure sensor applications, the sensor also includes an integrated full Wheatstone bridge for sensing temperature which is typically incorporated off the diaphragm to minimize its sensitivity to pressure. The temperature bridge typically is mechanized using two types of implant resistors, one type with a high temperature coefficient of resistance (TCR) for one set of diagonally opposite legs of the bridge and the other type with a low TCR for the other set of diagonally opposite legs of the bridge. The applied temperature imbalances the bridge, producing a differential, ratiometric output signal that is proportional to temperature and the bridge excitation voltage.
Additional implanted resistors can also be incorporated on-chip to provide bias and feedback gain resistors for connection to an external operational amplifier which then provides a single-ended amplified and signal conditioned output for both temperature and pressure. The temperature output can be used either as a stand alone temperature measurement, or for analog temperature compensation or, more commonly, used in microprocessor-based transducers to accurately calibrate and compensate the pressure sensor output over the full operating ranges of pressure and temperature.
In the pressure bridge configuration, the resistance of diagonally opposed legs varies equally and in the same direction as a function of the mechanical deformation caused by pressure. As the resistance of one set of diagonally opposed legs increases under pressure, the resistance of the other set decreases, and vice versa. Bridge excitation in the form of a voltage or current is applied across two opposite nodes of the bridge. These nodes are usually referred to as excitation inputs or bridge drive inputs. The piezo-resistor bridge differential output at the output nodes of the bridge with full scale pressure applied is equal to the product of the piezo-resistor gauge factor and the bridge excitation voltage, where the gauge factor is defined as the change in resistance due to the strain induced at full scale pressure conditions (ΔR) divided by the resistance (R) at zero pressure input conditions. Assuming that the magnitudes of (ΔR)/R of the bridge element are equal, the differential voltage (ΔV) at full scale pressure is expressed as follows: (ΔV)(@FS)=(ΔR/R)×Vbridge.
For silicon piezo-resistive sensors, the gauge factor (ΔR/R) at 25° C. may range from 0.03 to as high as 0.12 depending on the limitations of the application such as linearity and overpressure ratings. This range in gauge factor corresponds to full scale output ranges of from 150 mV to 600 mV with 5 volts bridge excitation, which is significantly greater (approximately 100 times) than typical metal strain gauge type sensors. However, the full span output (FSO) of an uncompensated piezo-resistive sensor can exhibit a strong nonlinear dependence on temperature caused be the intrinsic nonlinear dependence of the piezo-resistor gauge factor (ΔR/R) on temperature, whereas the zero pressure (null) offset and null offset dependence on temperature are maintained small in comparison.
The full scale span output is defined as the difference in sensor output corresponding to the maximum and minimum applied pressures. Span shift with temperature is defined as the span as a function of temperature divided by the span at 25° C. Span Shift(T) in percent is equal to 100·[Span(T° C.)/Span(25° C.)]. The span shift curve is nonlinear with a negative slope with temperature as illustrated in FIG. 4 and is identified in FIG. 4 as K3, where K3 is defined as the ratio of the pressure sensitivity (ΔR/R) of the heavy implant piezo-resistive bridge as a function of temperature normalized to the value at 25° C. In equation form, K3(T)=[(ΔR/R(T)]/[(ΔR/R(25° C.)] and may be expressed by the following 5th order polynomial:K3(T)=−(6.265753E−14)·T^5+(5.393845E−11)·T^4−(2.440481E−08)·T^3+(8.022881E−06)·T^2−(2.585262E−03)·T+(1.058300) The magnitude of the slope decreases with increasing temperature. A typical value of K3 at 25° C. is −0.25%/° C. Thus, in most applications, the sensor bridge output must be compensated, for the span shift(T) in particular, before it can be used in practice.
FIG. 4 also illustrates the temperature characteristics of the heavy implant resistors (K2) and the light implant resistors (K1). The heavy implant resistors (K2) is defined as follows: K2=Ratio of the resistance of the heavy implant as a function of temperature normalized to the value at 25° C. In equation form, K2=[Rheavy(T)]/[Rheavy(25° C.)] and may be expressed by the following 5th order polynomial:K2(T)=−(3.018497E−14)·T^5+(4.603604E−11)·T^4−(2.282857E−08)·T^3+(7.538750E−06)·T^2−(2.252834E−05)·T+(0.9963789) The light implant resistors (K1) is defined as follows: K1=Ratio of the resistance of the light implant as a function of temperature normalized to the value at 25° C. In equation form, K1(T)=[Rlight(T)]/{Rlight(25° C.)] and may be expressed by the following 5th order polynomial:K1(T)=−(8.171496E−14)·T^5+(9.930398E−11)·T^4−(3.557091E−08)·T^3+(9.691127E−06)T^2+(2.958093E−03)·T+0.923953 It is noted that the change in resistance of the light implant resistors as a function of temperature K1(T) is much greater than that of the heavy implant resistors K2(T).
In the temperature bridge configuration, the resistance of diagonally opposed legs varies equally and in the same direction as a function of temperature. As the resistance of one set of diagonally opposed legs increases more due to high positive temperature coefficient of resistance (TCR), the resistance of the other set increases less due to a low positive temperature coefficient of resistance (TCR). Bridge excitation in the form of a voltage is applied across two opposite nodes of the bridge. These nodes are usually referred to as excitation inputs or bridge drive inputs.
Therefore, piezo-resistive bridge pressure sensors frequently include signal conditioning and calibration circuits. For example, a high-gain, low-noise, temperature stable amplifier may be used to scale the output to more usable levels. The signal-conditioning circuit also typically includes span compensation. The total resistance and the piezo-sensitivity (the ratio of the bridge output to excitation voltage of the bridge) of piezo-resistive bridge pressure sensors are temperature dependent. Typically, bridge resistance increases with temperature while piezo-sensitivity decreases.
Moreover, present piezo-resistive pressure sensors provide only a single fixed light implant (high TCR) resistor element in order to set the gain (feedback) of the on-chip temperature bridge output. Accordingly, this resistor provides a specific value for a particular operating temperature range and, therefore, is not optimum for different or extended operating temperature ranges.
Also, present pressure sensors (designed for constant voltage operation) do not provide an integrated function for unique span compensation of the heavy implant (low TCR) pressure bridge when operated in the constant current mode. Nor do they provide an integrated voltage potential customized to be applied to the chip's epitaxial layer in Silicon-On-Insulator (SOI) applications in order to reduce warm-up drift effects and non-ratiometric errors of the pressure bridge output.
The pressure invention is intended to solve these or other problems.